Velvet underglazes behave like regular underglazes. They are best applied on greenware but will usually work on bisque as well. Fire to cone 06 or similar. No special brushes or techniques necessary.
I have no idea what the directions for using velvet underglazes are. I did ceramics a long time ago, but never used a velvet underglaze. I assume that you paint it on and fire it and then paint another glaze over it.
